Supercapacitors store ionic charge electrostatically at the interface of high surface area electrodes, such as carbon electrodes, in a liquid electrolyte composition. Supercapacitors are also referred to interchangeably as ultracapacitors or electric double-layer capacitors (EDLC). Efforts to increase the energy density of supercapacitors have focused mainly on developing higher surface area electrodes and controlling electrode pore size. Energy density of supercapacitors can also be increased through faradaic mechanisms commonly known as pseudocapacitance, which arises from the introduction of redox active groups through functionalization of the carbon electrode surface or the incorporation of metal oxides.
Despite significant improvements in electrode materials design, most non-aqueous electrochemical capacitors use the same electrolyte compositions: either a mixture of tetraethylammonium tetrafluoroborate (TEABF4) in acetonitrile (AN) or TEABF4 in propylene carbonate (PC). These electrolyte compositions have a high specific conductivity that minimizes resistive losses and enables capacitors to operate at high power. However, these electrolyte compositions typically exhibit a practical voltage window around only 2.5-3.0 V, beyond which the capacitor lifetime is significantly reduced. Ionic liquids and other organic solvents (adiponitrile, sulfones, and carbonates) have been considered for high voltage electrolytes for capacitors. Of these, ionic liquids have generated the most interest due to their high stability, but remain limited by high cost, low purity, and low conductivity. Since the energy stored in a capacitor increases quadratically with voltage, extending the electrochemical window of the electrolyte composition could significantly improve the energy density of the capacitor.
With prolonged cycling, the capacitance of EDLCs decreases and the resistance increases. The performance degrades more rapidly at elevated temperature or higher voltage. Degradation is typically attributed to decomposition of the electrolyte, and is very sensitive to the electrolyte composition, electrode polarity, carbon surface functionality, and trace moisture. The long-term performance of EDLCs can also be limited by the stability of other components in the cell including the carbon, polymer binders, and current collectors (typically aluminum). Commercial EDLCs with organic electrolytes operate over a voltage window between approximately 1.5 and 4.5 V vs. Na/Na+. Developing higher voltage electrolytes for EDLCs requires careful consideration and control of all possible side reactions. For example, extending the positive voltage limit beyond 4.5 V vs. Na/Na+ likely requires strategies to effectively suppress corrosion of the aluminum current collector. Carbon oxidation may also occur at high voltage.
Extending the negative voltage limit below 1.5 V vs. Na/Na+ also presents certain challenges. The solvents most commonly used in lithium-ion batteries and EDLCs (carbonates and ACN) passivate electrodes at potentials below about 1.2 V vs. Na/Na+. Effective passivation of the negative electrode is critically important for the operation of lithium-ion batteries, but detrimental for double-layer capacitors. Even very thin insulating surface films can reduce the double layer capacitance and block small pores. Binders based on polytetrafluoroethylene (PTFE), which are commonly used in commercial EDLC electrodes, are also reduced below about 1.0 V vs. Na/Na+. Finally, the stability of the carbon itself with respect to reduction and/or intercalation of cations must be considered. Accordingly, there remains a continued need for an electrolyte composition that can extend the operating voltage window of a supercapacitor.